1. Field of the Invention
The invention relates generally to the field of fiber optic telemetry systems and fiber optic sensor systems. More specifically, the invention relates to wavelength division multiplex telemetry systems for an array of fiber optic sensors.
2. Background Art
Fiber optic sensors known in the art include an optical fiber sensing element, typically in the form of a coil of optical fiber, that is arranged such that the length and/or the refractive index of the optical fiber sensing element is changed by the action of the physical parameter being measured by the sensor. For example, a fiber optic acoustic sensor has a fiber sensing coil wound around a cylinder, the shape of which changes by the action of a pressure wave, or an acoustic wave, impinging on the cylinder. Deformation of the cylinder changes the length of the sensing coil. Light from a source such as a laser diode is passed through the sensing coil and is simultaneously passed through a “reference” element or coil of optical fiber. The reference element is arranged so that it is not physically affected by the physical parameter being measured. Light exiting both the sensing element and the reference element are then combined in any one of a number of types of optical interferometer. An optical interference pattern is generated in the interferometer which is related to the change in length of the sensing element (and thus the phase of the light passing through the sensing element). A photodetector is optically coupled to the output of the interferometer, and may be used to generate an electrical signal that corresponds to the intensity of light reaching the photodetector (which is related to the interference pattern), and thus corresponds to the physical parameter affecting the sensing element. In the case of a fiber optic acoustic sensor, the electrical signal from the photodetector corresponds to the change in pressure, or to the amplitude of the acoustic wave, impinging on the sensing coil.
Fiber optic sensors have proven very useful because of their relative immunity from electromagnetic interference, among other factors, particularly when signals are transmitted over long distances in optical form. Because of the relative advantages of fiber optic sensors, efforts have been made to substitute fiber optic sensors in applications such as marine seismic sensor arrays. Conventional marine seismic sensor arrays known in the art typically include a plurality of electrical-signal generating acoustic sensors, typically magnetostrictive or piezoelectric hydrophones, disposed at spaced apart locations along a reinforced cable adapted to be towed in a body of water. Typically, such a seismic sensor array includes various signal processing devices disposed at selected locations along the cable which detect, amplify, and digitize the electrical signals generated by each of the sensors. The digitized signals are included in electrical signal telemetry in order to transmit signals from each one of the sensors in such a way that the signal from each sensor can be uniquely identified, recorded and processed. Electrical telemetry systems known in the art are intended to minimize the number of electrical conductors needed to transmit signals from a selected number of sensors, and to minimize signal distortion between the sensors and a recording device. It is also known in the art to convert the digitized electrical signals into optical signals and use optical telemetry to transmit the optical signals from the electrical/optical conversion point to an optical signal receiver.
Optical sensing and signal telemetry are desirable to use in marine seismic sensor systems because, as previously explained, they are less susceptible to various forms of interference and signal loss along the cable. In order to more effectively substitute optical sensors for electrical sensors in a marine seismic sensor array, it is desirable to have an entirely optical telemetry system to transmit signals from each optical sensor to a seismic signal recording system. The telemetry system must enable unique identification and recording of the signals from each individual optical seismic sensor. One type of fiber optic sensor array and an optical telemetry system are disclosed in U.S. Pat. No. 6,285,806 B1 issued to Kersey et al . . . The sensor array disclosed in the Kersey et al. patent includes a plurality of fiber sensing coils optically coupled in series along an optical signal line. Each fiber sensor coil is followed along the signal line by a fiber Bragg grating adapted to reflect a portion of the light passing through it back through the optical signal line. Light from a source, such as a laser diode, is modulated into a pseudo-random binary (or bit) sequence (PBRS) and is coupled into the signal line. A code generator which produces the PBRS is also coupled to a time delay circuit. The signal line is also coupled to a photodetector. Groups of the sensors are interrogated by correlation of the light reflected back along the signal line by the Bragg gratings to the modulated input light. Correlation is performed with respect to the time delay added by the delay circuit. The time delay is specifically selected to interrogate length-wise segments of the signal line from the light input end to a selected endpoint at one of the sensing coils. The correlated signal includes the effects of all the sensors from the light input end to the selected endpoint. Individual sensor signals may be determined by subtraction of correlated signals representing different selected lengths of the signal line. A purported advantage of the array disclosed in the Kersey et al. '806 patent is that no optical couplings are needed between the individual sensors and the signal line, with attendant losses of light amplitude. The system disclosed in the Kersey et al. patent is specifically adapted to operate on a single wavelength of light.
Another type of sensor array is disclosed in U.S. Pat. No. 5,206,924 issued to Kersey, which includes a light source coupled to a polarization beam splitter. One output of the polarization beam splitter is coupled to one or more fiber optic sensors along a sensor line. The individual sensors are coupled to the sensor line in series, and each sensor is followed in series by an optical delay line. Each of the sensors includes a reference coil and a sensing coil. The input ends of each sensing coil and each reference coil are optically coupled to the signal line through an optical coupler. The output ends of each sensing coil and each reference coil are terminated in a Faraday rotator and a mirror. When light passes through the Faraday rotator, its polarization state is rotated 45 degrees. In the sensors of the array disclosed in the Kersey '924 patent, the mirror at each coil termination returns the light back through the same Faraday rotator, again rotating the polarization state by 45 degrees. Light returns back through the sensor coil and the reference coil having polarization states rotated by a total of 90 degrees before the light is returned to the sensor line through the optical coupling. The optical coupling performs the function of an interferometer. Interference patterns from the optical couplings along the sensor line are transmitted back to a photodetector coupled to one input of the polarization beam splitter. Individual sensors are interrogated by appropriate pulsing of the input light to create a time division telemetry scheme. A purported advantage of the array show in the Kersey '924 patent is substantial elimination of polarization fading along the signal line, without the need for expensive, high-birefringence “polarization preserving” fibers.
Yet another optical sensing system is disclosed in U.S. Pat. No. 5,140,154 issued to Yurek et al. The system disclosed in the '154 patent includes an inline fiber optic acoustic sensor array. The array includes first and second fiber optic sensor units for sensing a desired property. The first and second sensor units are connected in a linear array, such that each sensor unit incorporates a complete functional sensor, and the two sensor units are separated from each other by an intermediate delay element responsive both to the desired property being measured and to environmental stress connected between the first and second sensors for providing time separation between signals corresponding to the desired measured property produced by the first and second sensor units, and signals produced by the array in response to environmental stress impinging on the delay element. Each fiber optic sensor produces a modulated coherent light beam in response to an impinging desired measured physical property. The delay element produces a modulated coherent light beam in response to both an impinging desired measured property and environmental stresses. Modulated coherent light beams produced by each fiber optic sensor are time separated from modulated coherent light beams produced by the adjacent delay element. Conventional time discriminating signal processing techniques are used to interrogate only the sensor units, or to otherwise eliminate electrical signals corresponding to the modulated coherent light beam produced by the delay element, thus decoupling environmental stresses from the electrical signals being processed.
There continues to be a need to improve isolation of individual sensor signals in a fiber optic sensor array, to improve the amplitude and fidelity of the sensor signals returned to a recording unit, and to minimize the number of optical fibers needed in any particular fiber optic sensor array.